[Caml-list] Type variables (was: Odd Type Checking Problem)

[Alain Frisch <frisch@clipper.ens.fr>]

Hello,

On Wed, 6 Feb 2002, I wrote:

> I guess this is a problem of understanding type variable scoping rules.


Actually, I feel myself somewhat confused with implicit introduction and
scoping of type variables.

These one are refused:

let f (x : 'a) = let module M = struct exception X of 'a end in ();;
let f (x : 'a) = let module M = struct type t = 'a end in ();;

This is accepted:

let f (x : 'a) =
  let module M =
      struct
	type t constraint t = 'a;;
	exception X of t;;
      end in ();;

but is quite useless, since both:

let f (x : 'a) =
  let module M =
      struct
	type t constraint t = 'a;;
	exception X of t;;
	raise (X x);;
      end in ();;

and

let f (x : 'a) =
  let module M =
      struct
        type t constraint t = 'a;;
        exception X of t;;
      end in raise (M.X x);;


are rejected by the type checker.

(in "constraint ...", no new variable is introduced)


Another example:

let f (x : 'a) =
  let module M =
      struct
        type u = { a : 'a }
      end in ();;

=> rejected (Unbound type parameter 'a)

(and accepted by
"Objective Caml version 3.04+1 with explicit polymorphism (2002-01-07)",
but with a different meaning that one could expect)


Is there a way to use a type variable such as the 'a above to define
types in a local structure ?


-- Alain

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